The present invention refers to a string for rackets and, in detail, to a string for rackets used for sports such as tennis, badminton, and squash, etc., for which a synthetic resin fiber is used as a material.
It is known to produce a string by winding monofilaments made of synthetic resin about the periphery of a monofilament center core, also made of a synthetic resin. This type of a string, composed of a monofilament center core and a wrapping monofilament, can be improved in its important properties by choosing particular combinations of materials for the monofilament center core and the wrapping monofilament.
The properties required for a string for rackets comprise durability and repulsive force coupled with the ability to easily impart a spin or a slice to an object being struck. Also, to increase its commercial value, its external appearance should be beautiful.
Special features of the strings made by wrapping monofilament around a monofilament center core include the ability to produce a thick string, which is hard to do with a monofilament center core only, to be able to easily change the outer diameter of the string by selection of the outer diameter of the wrapping monofilament, to enhance such mechanical strength as durability, etc., to have the ability to control the imparting of a spin or slice to an object being struck because friction resistance between the string and the ball being hit increases with formation of unevenness on the peripheral surface of the composite due to the wrapping monofilament.
It has been sought to improve the above-described control property. Further, in a case where the string is strung on a racket, it has been found that, when hitting a ball, the filament is apt to snap due to rubbing between warp and weft strings at their crossing points. Improvement in strings to alleviate this problem is also desired. However, since the known wrapping monofilaments have been uniformly prepared from monofilament of the same outer diameter, improvement in the control property is very limited and prevention of rubbing between warp and weft strips is not sufficiently possible.
Thus, a string was proposed in which the monofilament center core had wrapped therearound mixed monofilaments of a small diameter and a large diameter, respectively. This string causes a part of the wrapping monofilament having a large diameter to project away from the core filament further than the diametrical projection of the small diameter wrapping filament, so that there are alternatively formed a generally convex area corresponding to the projected part of the large diameter wrapping monofilament and a generally concave area corresponding to the diametrical surface portions of the small diameter wrapping monofilament. If this string is strung on a racket, rubbing can be prevented at the above-described crossing point of the warp and the weft strings by contacting a raised (convex) portion of one string with a depressed (concave) portion of the crossing string. Also, this unevenness, due to the convex and concave areas, gives greater friction when a ball is hit, so as to more easily impart a spin or a slice.
However, a string for rackets prepared by winding a wrapping monofilament around the periphery of a monofilament center core is less than satisfactory in that its knot strength is small in comparison with its tensile strength.
The tensile strength is determined by a weight breaking a string when being drawn straight and the knot strength is determined by a weight breaking a string when both the terminal ends of a knot formed in the string are drawn apart. Ordinarily, the string breaks at the knot or knotted part when the string is employed for rackets. That is, when a string is strung on a racket frame lengthwise and crosswise, the knot strength is a better measure of the practical strength of the string than is the previously described tensile strength of the string.
In a string constructed of a wrapping monofilament around the periphery of a monofilament center core, when its knotted part is stressed, the wrapping monofilament is stretched more on its peripheral side than is the monofilament center core. Therefore, when the elongation is of the same order of magnitude for both the core monofilament and the wrapping monofilament, even if the core monofilament is not stretched so far that it still has sufficient strength to resist breaking, the wrapping monofilament having been stretched to a greater extent, does break. Normally, the wrapping monofilament is thin in comparison with the core monofilament and lower in the knot strength. Therefore, the knot strength of the whole composite string is limited by the knot strength of the wrapping monofilament.
On the other hand, in recent years the coloring and fashion-making for sport equipment, such as tennis rackets and so on, are in rapid progress, and regarding rackets, the design and color of racket frames, etc., have been widely varied. However, the strings for rackets have, in most cases, so far been simply transparent or of simple coloring and not satisfactory in terms of beauty and fashion.